(a) Field of the invention:
The present invention relates to display, and more particularly it pertains to light-emitting diode display capable of emitting a large amount of colored light.
(b) Description of the prior art:
At present, most colored light-emitting displays rely on a combination of incandescent lamps and color filters, or neon signs. However, the power efficiency of these conventional colored-light emission systems is very poor. Moreover, an incandescent lamp, wherein the light source relies on thermal radiation, requires a very high temperature at its light-emitting portion, and hence has a short service lifetime. Incandescent displays require replacement in a relatively short period of time. In case of traffic signal lamps which are to be used continuously, the durability of the respective light sources is less than one year.
Recently, semiconductor light-emitting diodes (LED's) have shown a marked improvement in their working efficiency and service lifetime. Such LED's have become readily available. However, semiconductor light-emitting diodes are presently employed only as indicators on control panels, such as of electronic devices, and they are not typically used in light-emitting devices, such as displays.
Semiconductor light-emitting diodes have been accepted in general as having a high brightness, but only a small amount of light emission per piece, and as being driven by a low dc voltage. Also, a light-emitting diode is typically manufactured from a compound semiconductor material having a broad energy gap. The LED has a threshold voltage value inherent to the semiconductor material employed. Such known diodes show a sharp increase in current in response to the application of a voltage above the threshold voltage value. As such, the driving of a known light-emitting diode requires the provision of a power (current) limiting means of some kind or other. For this reason, a simultaneous driving of a large number of light-emitting diodes requires complicated power supplies, resulting in a high cost of manufacture, and poor practical power efficiency. Thus, known semiconductor light-emitting diodes have been considered not suitable for use in ordinary light-emitting display devices excepting for special purposes.
Recently, however, as stated above, the power efficiency and the service lifetime of light-emitting diodes have come to supersede considerably those of incandescent lamps. For example, a red light-emitting diode formed of gallium-aluminium arsenide (available from Stanley Electric Co. Ltd., Japan) can provide 650 nm lights of above 2.35 cd/watt. Especially, in case of colored-light display, an incandescent lamp is such that only a small part of the total amount of light emission can be utilized, whereas a light-emitting diode is capable of emitting light rays of only effective wavelengths. Thus, the relative efficiency of light-emitting diode is great as compared to the incandescent lamp.
However, the abovementioned factors have hindered the materialization wide range use of light-emitting diode displays, i.e. the problems of power supply source, and cost.
In order to drive a visible-light emitting diode, a forward voltage of about 1.5 V up to about 4 V (which is determined depending on the wavelength of emitting lights and the semiconductor material employed) is required. At an excessively low voltage, the diode will not emit light, whereas an application of an excessively high voltage will cause an overflow of current, leading to destruction of the diode. Thus, it is not possible to drive a diode or diodes directly by the most commonly used power sources such as commercial power supply source, batteries for automobiles and power generator for bicycles.